1. Field of the Invention
The present invention relates to a three-group zoom lens adapted to form an image on a CCD or CMOS imaging device, and more particularly to a simple-structured variable-power imaging optical system to be suitably used for a digital or video camera.
2. Description of Related Art
The digital cameras, recently in rapid spread, use three-group zoom lenses in order to achieve compactness and aberration correction. Particularly, frequently used are the rear-focusing three-group zoom lenses adapted to move the final lens group forward (e.g. JP-A-10-133115, JP-A-2001-296476 and JP-A-2005-84597).
JP-A-10-133115 discloses a three-group zoom lens which includes a first lens group having a negative refractive power, a second lens group having a positive refractive power and an aperture diaphragm, and a third lens group having a positive refractive power, in order from the object side. When zooming from the wide-angle position to the telephoto position, the first lens group is moved toward the image side and then reversed toward the object side so that a moving path of the first lens is an arc that is convex toward the image side. The second lens group is moved toward the object side monotonously. The third lens group is moved toward the object side and then reversed toward the image side so that a moving path of the third lens is an arc that is convex toward the object side.
JP-A-2001-296476 discloses a three-group zoom lens which includes a first lens group having a negative refractive power, a second lens group having a positive refractive power and a third lens group having a positive refractive power, in order from the object side. In focusing at infinity during zooming from the wide-angle position to the telephoto position, the third lens group is moved toward the image side monotonously or depicting a path that is convex toward the image side.
Meanwhile, JP-A-2005-84597 discloses a three-group zoom lens structured smaller in size, higher in variable-power and in resolution as compared to those of JP-A-10-133115 and JP-A-2001-296476. This provides a structure that, by moving the third lens group toward the object side so that a convex arc is depicted during zooming, the curvature of field can be corrected well even with a middle magnification where curvature of field is difficult to correct at a high ratio of magnification.
However, in the background art, zooming is by moving the three lens groups thus making complicate the structure and operation of the mechanism.
Recently, plastic lenses are frequently used in an attempt to reduce the cost or weight, as described in JP-A-2005-84597. On the other hand, the use of plastic lenses causes deteriorated aberrations, including spherical aberration and lateral color, resulting from a change of environment, such as temperature or humidity.
Furthermore, in the background art, a lens in focusing is moved forward great in amount. Thus, there has been a desire for a three-group zoom lens capable of reducing the device thickness by reducing the movement of the focusing lens group and hence the lens overall length.